The invention relates to a method for transporting and erecting or replacing a unit, formed by the nacelle and the rotor, of an offshore wind turbine. The invention further relates to a watercraft for carrying out this method.
Wind power has already been known since antiquity to be used for driving machines, and nowadays it is of increasing relevance in particular for producing electrical power. The wind turbines that have evolved from the technology developed for windmills can be used at practically all sites for producing power, wind farms being installed to an increasing extent on the open sea and off the coast—that is to say offshore.
Offshore wind turbines find greater acceptance in the communities. They give fast-growing cities that are situated next to the sea the possibility that is environmentally and resource friendly, to safely guarantee the power requirement of the communities and industries.
Because of the corrosive behavior of sea water, particularly high demands are being placed on the materials used for wind turbines. Designs have to be used that can both withstand brief high energy gusts of wind and high waves.
Erection, maintenance and repair of an offshore wind turbine furthermore places high demands on the construction of the turbine and the transport and installation vessels required for this purpose. In adverse weather conditions, this work on the high seas can possibly be carried out not at all or only to a limited extent. It is therefore necessary especially for requisite larger repair work to be carried out very fast.
To this end it has already been suggested that wind turbines developed for offshore use are first assembled close to the coast to form functional or essentially functional wind turbines and then transfer them to that location where the wind turbine is to be used permanently. WO 01/34977 A1 for example describes such a method and installation resp. transport system for transporting a completely assembled wind turbine into an offshore area resp. out from there again into an area close to the shore for maintenance purposes.
However, the disadvantage of the transport method described in WO 01/34977 A1 is that the wind turbine depicted there already is of very large dimensions because of the assembly that has already taken place, in part it is very tall and also possesses a large lateral extent due to the rotor blade diameter. This entails problems in particular if there are obstacles during the course of the passage en route between that location where the wind turbine is assembled and the offshore area where the wind turbine is to be erected, such as bridges, locks etc. that make transport impossible due to the height and width dimensions of the wind turbine that is assembled and forms a functional unit. This further requires particularly large and thus expensive transport ships.
Most of today's wind turbines are designed such that individual components of the turbine can be replaced. This is a good solution for parts that are small and light-weight. However, if large and heavy components are involved, e. g. gearbox or generator, in-situ replacement or repair is extremely difficult to carry out. A floating crane or a jackup ship with the individual parts and a crew has to brought to the turbine. The components have to be removed under difficult conditions using non-optimal tools and lowered onto the transport ship. The new components have to be lifted into the nacelle and installed again. To all intents and purposes, the new components have a weight of 30 to 60 t, and installation requires a high degree of precision. This sort of repair procedure entails high costs for the transport vessel and the crew, power being lost at the same time due to the turbine standing still. Costs of some 40 to 60000 Euros can arise for each day of repair. Larger repairs may well take several days or even a week. There is then also the danger of the weather changing which may lead to the repair work having to be aborted and thus to the costs increasing even further.
This procedure can only be carried out very fast and thus cost-effectively if the rotor is already attached to the nacelle and can be lifted in one lifting operation on to the tower of the turbine. A three-blade rotor, that is conventionally used today, is not suited for such a procedure for reasons of geometry. Here the turbine has to be installed in two lifting operations for the nacelle and for the rotor. The installation is particularly difficult due to the fact that the three-blade rotor has to be stored on the transport ship in a horizontal position and then has to be rotated through 90° during the course of the lifting operation. This is a difficult operation that can also endanger the crew at higher wind speeds and can lead to the blades being damaged. Connecting the rotor with the nacelle likewise is a critical operation, in particular for offshore wind turbines. Here the hub bolts have to be inserted into the shaft flange and bolted on. This can easily lead the bolts or the bearing surfaces being damaged because at higher wind speeds the rotor cannot be guided precisely to its destination in the case of offshore installation processes.
The efforts, the costs, and the risks have to be regarded as very high for the offshore assembly of a three-blade rotor.